Marvel's Avengers universe continues down the TV episode route with this frantic, overly structured movie that essentially strings together chaotic, gratuitous action set-pieces with an offbeat but ultimately thin plot. It's enough of a spectacle to be watchable, even if writer-director Whedon relies far too much on digital animation.

After recapturing an important Asgardian sceptre, Tony and Bruce (Downey and Ruffalo) use the technology to boost Ultron, their plan for artificial intelligence peacekeeping. But with a mind of his own, Ultron (Spader) sets out to cleanse the planet. So the rest of the Avengers (Evans, Johansson, Hemsworth and Renner) must join them to stop Ultron, running into supercharged twins (Taylor-Johnson and Olsen) as they dart from America to Africa to Korea to Eastern Europe. And along the way, Tony's computer sidekick Jarvis (Bettany) evolves into an intelligent physical entity called The Vision.

This excellent cast shines in brief moments between the carnage, when the script allows them to develop their relationships and reveal more about themselves. There are also old friends (Jackson and Smulders), returning teammates (Don Cheadle, Anthony Mackie, Stellan Skarsgard), some sparky new figures (Linda Cardellini, Andy Serkis, Thomas Kretschmann) and flashback appearances (Idris Elba, Hayley Atwell, Julie Delpy). Yes, this is a seriously crowded movie, and Whedon does a remarkable job of keeping everyone straight while letting each actor have his or her moment to shine.

But it's impossible to escape the fact that he's even more interested in the action mayhem, which is shot and edited with a jarring harshness. There's also the increasingly annoying fact that this is an ultra-violent movie constrained by a PG-13 rating, so thousands of robots are torn to pieces while only a few people die. Which is frankly ludicrous considering the fallout of each battle sequence, most of which could be cut out entirely without changing the story.

After the fresh tone and plot of last year's Guardians of the Galaxy, this film feels surprisingly stale. Although die-hard fans will love every moment, as they spot geeky details that mean something larger in this universe. But the best scenes here are the quieter interpersonal moments, which are beautifully played with a hint of unexpected darkness. These simpler, more resonant things are what make the audience gasp. And what brings them back for more. So please break the formula again, because straining for more bombastic ways to save the world every time is just getting boring.